The present invention relates in general to a method for acoustic well logging and, more particularly, to a method for distinguishing between total subsurface formation permeability and that portion of such total subsurface formation permeability that can be attributed to naturally occurring fractures in the subsurface formation.
It is well known that measurements of the conductivity and porosity of a formation are important in determining whether hydrocarbons are located therein, the hydrocarbon being found as non-conductive fluids in porous rock formations. It is also well known that hydrocarbons are generally not extractable from porous rock formations unless those formations are also permeable. At the present time, no apparatus is known for accurately measuring the permeability of a rock formation in-situ. Accordingly, after hydrocarbons are detected, it has been generally necessary to obtain a side wall core of a given formation in order to measure permeability in the laboratory. Such a technique is time consuming and unduly expensive.
Techniques of acoustic well-logging are also well known and the possible applicability of such techniques to the determination of the permeability of a formation was predicted by Rosenbaum in the article "Synthetic Microseismograms: Logging In Porous Formations", Geophysics, Volume 39, No. 1 (February 1974). Rosenbaum investigated numerically the ideal case of a borehole filled with an inviscid fluid surrounded by a formation that is porous and which conforms to Biot's theory. His investigation showed that the effect of the pore-fluid mobility on the calculated response is large and can be measured with an appropriate acoustic or sonic logging tool. He further predicted that the relative amplitude of a tube wave obtained from a sonic tool with a wide band frequency response would depend upon the permeability of the formation.
Reacting to the predictions of Rosenbaum, at least one attempt has been made to employ standard sonic logging tools for the determination of permeability. In a paper entitled "Permeability Profiles From Acoustic Logging", by J. J. Staal and J. D. Robinson, presented to the 52nd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of A.I.M.I. held in Denver, Colo., Oct. 9-12, 1977, it was reported that with a standard Schlumberger sonic probe having transmitter-receiver spacing of up to 5 feet, a correlation could be detected between permeability and tube wave attenuation.
While standard sonic logging tools are available for permeability determination, a need still exists for improved techniques for permeability determination.